'a throw is legal as long as part of the foot is touching the boundary line at the point of delivery' offers clarity
It might offer clarity but it is not correct - it only covers the first part of the wording for foot positioning and the wording in the law - "part of each foot on the touchline" means the same as "
part of the foot is touching the boundary line" but uses fewer and simpler words (also "each foot" is clearer and more accurate than "the foot").
However more importantly, it does not cover the second part of the wording "or on the ground outside the touchline" which means that a throw-in can still be legal even if none of either foot is touching the line but is outside the line.
The fact is that the current wording states in simple and concise language exactly what is required in terms of foot position to make a throw legal. If either or both of the simple requirements it lays down are met, the throw is fine. If they are not met, the throw is not legal. I have to say I am at a bit of a loss as to understand the difficulty in seeing this. As it stands, the law is already clear and does not really need changing to make it any clearer, as far as I can see.
Perhaps it would help to take a step back and look at it again, thinking through the exact meaning and implications of each word individually and in combination. How about this - try looking at the two conditions as separate entities and imagine all the possible foot positions that would meet each condition separately (specifically, consider that the "part of each foot on the touchline" condition can still clearly met even if part of the foot is on the field of play) then consider the fact that if each foot is meeting one or other condition, the throw is OK. I think once you do that, you might hopefully see that the law is a model of brevity and clarity. It states exactly what it needs to say with no superfluous wording and covers all the possible permutations of correct foot positioning.
The only thing that would potentially make it even clearer (for me) would be if it actually stated that if part of either foot is on the field of play, the throw is still legal so long as part of each foot is on the touchline or on the ground outside the touchline as well. But it actually doesn't need to say that - as mentioned earlier, that meaning is inherent in the words already used.
Incidentally, this will be my last contribution on this topic, I doubt if I can really say anything else useful about it and I think the discussion has already gone on long enough (probably too long, in truth).